Extrusion method and extruder

ABSTRACT

In an extrusion method and an extruder, an annular material flow is created by extruding material into an annular extrusion channel ( 6 ). The extruder has a tubular filtering unit ( 7 ) arranged to the annular extrusion channel ( 6 ), whereby the annular material flow flows through the tubular surface of the filtering unit ( 7 ).

[0001] The invention relates to an extrusion method, in which method an annular material flow is created by extruding material into an annular extrusion channel.

[0002] The invention further relates to an extruder having means for extruding material and an annular extrusion channel, the means for extruding material being arranged to extrude the material into the annular extrusion channel so as to create an annular material flow in the extrusion channel.

[0003] In extrusion production, such as the manufacture of films and cables, it is often necessary to filter the plastic flow. Typically, the filter is located after an extruder before a press head and nozzles in a flow channel where the plastic flow is in a rod-like form. So as not to have to interrupt the flow during a filter change, a solution has been developed in which the filter has a filter tape that is fed substantially continuously in a transverse direction to the flow.

[0004] Extruders are also known that create an annular output flow, i.e. the output flow is created into an annular extrusion channel. EP patent 0 422 042, for instance, describes a conical extruder with which an annular output flow is created, whereby a tubular product can be formed without requiring a spider leg that causes weld lines in the product. If in this kind of solution, the plastic flow were to be filtered, the material would have to be made to flow in a rod-like form at some stage, which would remove some of the advantages provided by a conical extruder. Thus, filtering is not used in extruders that create an annular output flow.

[0005] It is an object of the present invention to provide a solution, in which in an extruder that creates an annular output flow, the material flow can be filtered without substantially weakening the properties of the final product.

[0006] The method of the invention is characterized in that in connection with an annular extrusion channel, there is a tubular filtering unit, and material is filtered by making it flow through the surface of the tubular filtering unit.

[0007] Further, the extruder of the invention is characterized in that the extruder has a tubular filtering unit that is arranged to an annular extrusion channel so that an annular material flow flows through the tubular surface of the filtering unit.

[0008] An essential idea of the invention is that an output flow is created into an annular extrusion channel in the extruder, and that the extruder has a tubular filtering unit arranged to the annular extrusion channel so that an annular material flow flows through the tubular surface of the filtering unit. The idea of one preferred embodiment is that the filtering unit has at least three layers, in which case the outer and inner layers of the filtering unit are substantially stationary and have between them a layer that can be moved. The outer and inner layers have holes and the material is arranged to flow sideways in the intermediate layer, whereby it is filtered and the filtered material remains in the layer in question on the side of the holes of the layer first in the flow direction of the material. When the filtering unit needs to be cleaned, the intermediate layer is moved in such a manner that the filtered material settles at the holes of the layer last in the flow direction of the material and due to the material flow, the filtered material particles flow out of the extruder. For a time, unsuitable material then exits the extruder, but the filtering unit is substantially clean and can efficiently filter new material. Thus, when cleaning the filtering unit, its inlet part is changed into its outlet part and vice versa. The idea of one preferred embodiment is that the extruder has at least two feed gaps within each other and shaped like a convergent cone, and at the end of a rotor located between the feed gaps, a return channel is formed extending a distance backwards from the end of the rotor. Both feed gaps allow a small amount of material to flow into the return channel, whereby an uncontrolled mixing of materials in different feed gaps or their burning to the filtering unit or to the end of the rotor is prevented.

[0009] The invention provides the advantage that the extruder provides an annular material flow that can be kept annular even though the material flow is filtered. The filtering surface area formed by the tubular filtering unit is very large, and the resistance caused by the filtering unit remains fairly small even though filtering efficiency is good. In a three-layer filtering unit, by changing the inlet part of the filtering unit to be the outlet part and vice versa, the filtering unit can be cleaned, i.e. the extruder can be a continuous one, because the blocking of the filtering part or the need for maintenance do not cause a break in the operation of the extruder. Arranging a return channel to the end part of the rotor provides a structure that makes it possible to extrude two different plastic materials in different feed gaps of a conical extruder and to filter them without an uncontrolled mixing of the materials or without their burning to the end of the extruder.

[0010] The invention is described in greater detail in the attached drawings, in which

[0011]FIG. 1 is a schematic cross-sectional side view of an extruder,

[0012]FIG. 2 is a schematic cross-sectional side view of the end part of an extruder,

[0013]FIGS. 3a and 3 b are schematic cross-sectional views of a filtering unit in the axial direction,

[0014]FIG. 4 is a schematic cross-sectional view of a second filtering unit in the axial direction,

[0015]FIG. 5 is a schematic cross-sectional view of a third filtering unit in the axial direction,

[0016]FIG. 6 is a schematic cross-sectional view of a fourth filtering unit in the axial direction,

[0017]FIG. 7 is a schematic side view of the placement of holes in the filtering unit shown in FIG. 6,

[0018]FIG. 8 is a schematic cross-sectional view of a fifth filtering unit as seen diagonally from the top,

[0019]FIG. 9 shows a schematic representation of a sixth filtering unit, and

[0020]FIG. 10 shows a schematic cross-sectional side view of the end part of a second extruder.

[0021]FIG. 1 shows an extruder having a conical inner stator 1 and a conical outer stator 2. A rotatable rotor 3 is arranged between the stators 1 and 2. An inner conical feed gap 4 forms between the inner stator 1 and the rotor 3 and an outer conical feed gap 5 is between the outer stator 2 and the rotor 3. Either the rotor 3 or the stators 1 and 2 or both have helical grooves, whereby when the rotor 3 is rotated, material flows from the feed gaps 4 and 5 into an annular extrusion channel 6. For clarity's sake, FIG. 1 does not show the grooves in the rotor and/or stators, or a rotating apparatus of the rotor or feeding means for feeding material to be extruded into the feed gaps 4 and 5, because said means and other corresponding details related to an extruder are obvious to a person skilled in the art and need, therefore, not be explained in detail herein. The material to be extruded is typically plastic, but can also be some other extrudable material. The extruder can be used for instance to manufacture a pipe, film, cable sheath or some other corresponding tubular product.

[0022] The extruder further has a tubular filtering unit 7 that is arranged in the extruder in such a manner that an annular material flow flows in from outside and is filtered and remains annular. The tubular filtering unit 7 is then preferably cylindrical or in the shape of a cone expanding in the flow direction of the material. The tubular filtering unit 7 can also be arranged in the extruder in such a manner that the annular material flow flows through it from inside to outside, in which case the tubular filtering unit 7 is most preferably in the shape of a cone converging in the direction of flow.

[0023]FIG. 2 shows a solution in which the extruder has an intermediate stator 8 between an inner stator 1 and outer stator 2. Between the inner stator 1 and the intermediate stator 8, there is an inner rotor 3 a, and between the intermediate stator 8 and the outer stator 2, there is an outer rotor 3 b. The rotors 3 a and 3 b have helical grooves 9. In addition to or instead of the grooves in the rotors 3 a and 3 b, the stators 1, 2 and 8 may also have corresponding grooves.

[0024] The tubular filtering unit 7 has three layers. The outermost layer is an outer perforated layer 7 a and the innermost layer is an inner perforated layer 7 c. Between them, there is a filtering layer 7 b. The structure and operation of the filter 7 is described in more detail in FIGS. 3a and 3 b. The outer perforated layer 7 b has inlet openings 10 and the inner perforated layer 7 c has outlet openings 11. The inlet openings 10 and the outlet openings 11 are arranged at different points in such a manner that in the filtering layer 7 b, material has to flow sideways as shown by the arrows in FIGS. 3a and 3 b. The fact that the material flows sideways in the filtering layer 7 b means that the material can flow in the direction of the cylindrical or conical surface of the filtering unit 7. The material can thus flow in the circumferential direction or in the axial direction or in any direction between these two. The filtering layer 7 b has first openings 12 a that in the case of FIG. 3a are located at the inlet openings 10, and second openings 12 b that in the embodiment of FIG. 3a are arranged at the outlet openings 11. Thus, the material flows first into the inlet opening 10 of the outer perforated layer 7 a and from there to the first opening 12 a of the filtering layer 7 b. After this, the material to be extruded flows through the filtering material of the filtering layer 7 b which filtering material is made such that it allows particles smaller than an certain size to pass, i.e. main part of the material to be extruded can flow through the filtering material, but impurities and too large particles remain in the first opening 12 a. After flowing through the filtering material, the material enters the second opening 12 b of the filtering layer 7 b and flows on through the outlet opening 11. The filtering material of the filtering layer can be sintered metal powder, for instance.

[0025] At the stage when impurities and/or large particles have accumulated in the first opening 12 a of the filtering layer 7 b to the extent that they begin to hamper the material flow, the filtering layer 7 b is shifted in such a manner that the first openings 12 a coincide with the outlet openings 11 and the second openings 12 b coincide with the inlet openings 10 as shown in FIG. 3b. The material flow then pushes the impurities and/or too large particles through the outlet opening 11 into the extrusion channel 6 and consequently, the filtering unit becomes clean. Due to this so-called phase shift, the flow direction in the filtering layer changes, but because of the structure of the filtering unit, the extruder can operate continuously. Naturally, for a time after the shift, the material exiting the extruder contains impurities and/or too large particles, i.e. the product is probably unsuitable. This cleaning action can, however, be done for instance at the time of a roll change or during some other suitable time in production when unsuitable material can be led to rejects.

[0026] As stated above, the operation of the extruder can be kept continuous and good-quality material obtained quite quickly. Moving, i.e. rotating, the filtering layer 7 b can be done by a rotating apparatus 13. The rotating apparatus 13 can, for instance, be a cogwheel, screw or some other means suitable for the purpose. The distance between the first openings 12 a and the second openings 12 b and, on the other hand, the distance between the inlet openings 10 and the distance between the outlet openings 11 must naturally be made suitable so that all openings coincide in a suitable manner both in the situation shown in FIG. 3a and in the situation shown in FIG. 3b. For cleaning the filtering unit, the filtering layer 7 b can moved onward, i.e. rotated, each time, or in consecutive situations, the filtering layer 7 b is moved into different directions, in which case the filtering layer is only moved back and forth and not rotated around. When the filtering layer 7 b has been shifted and impurities and/or large particles have detached from the filter and exited the extruder, normal production can be continued immediately, which means that the direction of material flow in the filtering layer 7 b changes each time after a cleaning operation. Another alternative is to move the filtering layer 7 b back to its original place after the cleaning operation, in which case in normal production, the material always flows in the same direction in the filtering layer 7 b. Turning the material flow only for cleaning ensures that particles weakening the properties of the final product cannot by accident enter the final product during normal production.

[0027] When the filtering unit 7 is substantially cylindrical, the filtering layer 7 b can be moved in the circumferential direction or in the axial direction for the purpose of cleaning. By making the openings 10, 11, 12 a and 12 b coincide suitably, the filtering layer 7 b can be moved in any direction between the circumferential and axial for the purpose of cleaning. The filtering layer 7 b can thus be made to move along an oblique or helical line, for instance.

[0028] Clearance adjustment rings 14 have been arranged to the forward and tail ends of the outer perforated layer 7 a and the inner perforated layer 7 c. By adjusting the size of the clearance adjustment rings, it is possible to easily alter the location of the filtering unit 7 in the axial direction, and since the filtering unit 7 is in the shape of an expanding cone, the clearance between the filtering unit 7 and the extruder can be adjusted at the same time. The clearance between the outer perforated layer 7 a and the filtering layer 7 b and, correspondingly, the clearance between the inner perforated layer 7 c and the filtering layer 7 b can be adjusted by altering the location of the outer perforated layer 7 a and the inner perforated layer 7 c in the axial direction by means of the clearance adjustment rings 14. A corresponding clearance adjustment can also be done when the tubular filtering unit 7 is in the shape of a convergent cone. Further, when the filtering unit 7 is conical, it is easier to match, assemble and disassemble its different layers than those of a cylindrical filtering unit.

[0029] For replacing the filtering unit 7, the tail end 2 a of the outer stator 2 can be detached from the outer stator and correspondingly, the tail end 1 a of the inner stator 1 can be detached from the inner stator. Most preferably, the filtering unit 7 is made such that it can in its entirety be replaced in one piece. The replacing of the filtering unit can then be done quickly, because the new filtering unit can be assembled and cleaned to wait for replacement. A guide piece 15 for guiding the material flow in the extrusion channel 6 can be fixedly connected to the inner perforated layer 7 c, in which case the guide piece 15 is replaced together with the filtering unit.

[0030] A pressure drop occurs in the axial direction in the filtering unit 7; this is marked δP in FIG. 2. In FIG. 2, the pressure is then higher in the lower parts than in the higher parts. Because of the pressure drop δP in the filtering unit 7, there is an upward material flow inside the filtering material in the axial direction in FIG. 2 that prevents the material from burning to the end of the extrusion apparatus.

[0031]FIGS. 4 and 5 show an alternative filtering unit structure. In FIGS. 4 and 5, the layers of the filtering unit 7 are shown horizontal, even though in reality the layers naturally are curved because the filtering unit 7 is tubular. In the embodiments of FIGS. 4 and 5, the filtering layer 7 b is made of a solid material that does not allow the material being extruded to pass through it. The filtering layer 7 b can be solid metal, for instance. The filtering is in this case done in such a manner that the clearances 16 between the outer perforated layer 7 a and the filtering layer 7 b and between the inner perforated layer 7 c and the filtering layer 7 b are arranged such that the material being extruded can flow through the clearances 16, but impurities and/or too large, particles remain close to the inlet openings until the filtering layer 7 b is shifted for the purpose of cleaning the filtering unit as described above. The size of the clearance 16 is naturally selected according to the desired filtering capacity. The size of the clearance 16 can be approximately 1 mm, for instance, when extruding a material containing reprocessed plastic. The solution of FIG. 5 corresponds quite a lot to that of FIG. 4, but in the solution of FIG. 5, the filtering layer 7 b has an uneven surface. Grooves, for instance, can be made by a lathe on the filtering layer 7 b. Due to the uneven surface, the filtering surface area can be increased as compared with the solution in FIG. 4. If necessary, the surfaces of the outer perforated layer 7 a and the inner perforated layer 7 c can also be made uneven.

[0032]FIG. 6 shows a solution in which the inlet openings 10 of the outer perforated layer 7 a are made in the shape of a convergent cone and the outlet openings 11 of the inner perforated layer 7 c are made in the shape of an expanding cone. The inlet openings 10 in the shape of a convergent cone guide the material fluently into the openings of the filtering layer 7 b. The outlet openings 11 in the shape of an expanding cone make it possible for the material to flow fluently on from the outlet openings 11. The solution of FIG. 6 makes the flow resistance of the filtering unit very small and there are no blind spots in the structure of the filtering unit where the material to be extruded could accumulate.

[0033]FIG. 7 is a schematic side view illustrating the staggering of the inlet openings 10 and the outlet openings 11. By arranging for instance the inlet openings 10 as shown in FIG. 7 in such a manner that the inlet openings above the horizontally adjacent inlet openings 10 are in the middle of the lower openings, several holes can be arranged in the filtering unit 7 and consequently, the flow resistance of the filtering unit can be kept quite low.

[0034]FIG. 8 shows an embodiment in which the openings 10, 11, 12 a and 12 b are elongated. In addition, the inlet openings 10 are convergent and the outlet openings 11 are expanding. Such a structure also ensures efficiently that the material to be extruded does not stick to the filtering unit 7.

[0035]FIG. 9 shows a cylindrical filtering unit 7 in which the openings 10, 12 a, 12 b and 11 are arranged in an oblique position with respect to the axial direction. For clarity's sake, FIG. 9 only shows a part of the openings 10, even though the filtering unit 7 naturally has openings 10 substantially on the entire surface of the filtering unit 7. The openings 10 are arranged obliquely in the same direction with respect to the rotation direction ω of the extruder rotors 3. The material flow then runs through the filtering unit 7 as efficiently as possible, i.e. the pressure loss caused by the filtering unit 7 in the material flow is as small as possible.

[0036]FIG. 10 shows a solution for making multi-layer products. The solution in question can, for instance, make a three-layer product in such a manner that by means of the outer rotor 3 b the material is extruded to form the outermost layer of the product. The material to form the innermost layer of the product is extruded in the inner feed gap 4 a of the inner rotor between the inner rotor 3 a and the inner stator 1. The outer feed gap 5 a of the inner rotor between the inner rotor 3 a and the intermediate stator 8 can extrude adhesion plastic, for instance, for attaching the outermost and innermost layers of the final product to each other. There is a clearance 17 between the inner rotor 3 a and the outer perforated layer 7 a of the filtering unit 7. Further, there is a ring channel 18 at the tail end of the inner rotor 3 a. Material flows from both the inner feed gap 4 a and the outer feed gap 5 a through the clearance 17 to the ring channel 18. One or more return channels 19 lead from the ring channel 18 to the outer feed gap 5 a at a distance from the inner rotor 3 a tip. Owing to the clearance 17, the end of the inner rotor 3 a does not chafe the filtering unit 7. Owing to the clearance 17, the ring channel 18, and the return channel 19, a small amount of the material fed through the inner feed gap 4 a and a small amount of the material fed through the outer feed gap 5 a are allowed to flow between the rotor 3 a and the filtering unit 7 to the return channel 19 and the materials do not burn to the end of the rotor or mix in an uncontrolled manner with each other, but said small amounts flow to the outer feed gap 5 a at a distance from the end of the rotor 3 a.

[0037] If the material to be extruded through the inner feed gap 4 a did mix in an uncontrolled manner with the adhesion plastic, the adhesion plastic would not function, i.e. the outermost and the innermost layer would not attach to each other sufficiently well. Because the materials flow to a distance from the tip of the rotor 3 a, said small material flow can be mixed with the adhesion plastic and the adhesion properties of the adhesion plastic will not weaken substantially. Most preferably, the return channel 19 leads according to FIG. 10 to the outer feed gap 5 a, through which adhesion plastic is fed, because the consumption of adhesion plastic does not then increase, but instead decreases as compared with a situation in which said return channel does not exist. If the return channel led to the inner feed gap 4 a, more adhesion material would be consumed as compared with the abovementioned situation. At least the end part of the outer feed gap 5 a is preferably made such that the surface between the rotor and stator is wave-formed, most preferably saw-tooth-waved, i.e. stepped. The grooves 9 are then made such that their depth varies in such a manner that from the rotor groove the material moves substantially in its entirety into the stator groove and vice versa, and the interface between the rotor and stator cuts the material flow. This type of a structure forms a powerful mixer producing positive pressure. To make back flow possible, pressure P1 at the end of the rotor 3 a must be higher than pressure P2 in the end part of the return channel at a distance from the end part of the rotor 3 a.

[0038] The drawings and the related description are only intended to illustrate the idea of the invention. The invention may vary in detail within the scope of the claims. Thus an annular output flow can also be created by other than conical extruders. The annular output flow can thus for instance be created by an extruder in which a material feed apparatus feeds material through a radial pump unit made up of conical cogwheels, as described in WO publication 00/23251, for instance. 

1. An extrusion method in which method an annular material flow is created by extruding material into an annular extrusion channel (6) comprising a tubular filtering unit (7), whereby material is filtered by making it flow through the surface of the tubular filtering unit (7), characterized in that the filtering unit (7) has at least three layers, and the outer and inner layers (7 a, 7 c) of the filtering unit (7) are substantially stationary and have between them a filtering layer (7 b) that can be moved, and the outer and inner layers (7 a, 7 c) have openings (10, 11) and the material is arranged to flow sideways in the filtering layer (7 b), whereby it is filtered and the filtered material remains in the filtering layer (7 b) on the side of the openings (10) of the layer (7 a) first in the flow direction of the material, and the filtering unit (7) is cleaned by moving the filtering layer (7 b) in such a manner that the filtered material settles at the openings (11) of the layer (7 c) last in the flow direction of the material and due to the material flow, the filtered material particles flow out of the extruder.
 2. A method as claimed in claim 1, characterized in that the annular material flow is created by an extruder having at least one conical stator (1, 2, 8) and at least one conical rotor (3, 3 a, 3 b).
 3. A method as claimed in claim 2, characterized in that the extruder has at least two stators (1, 2, 8), and there is a conical feed gap (4, 4 a, 5, 5 a) on both sides of the rotor (3, 3 a, 3 b) and there is at least one return channel (19) at the end of the rotor (3, 3 a, 3 b), whereby material flowing through the feed gaps (4, 4 a, 5, 5 a) can flow through the return channel (19) from the end of the rotors (3, 3 a, 3 b) backwards to a distance from the end of the rotor (3, 3 a, 3 b).
 4. An extruder having means for extruding material and an annular extrusion channel (6), and the means for extruding material are arranged to extrude the material into the annular extrusion channel (6) in such a manner that an annular material flow is created in the extrusion channel, the extruder further comprising a tubular filtering unit (7) arranged to the annular extrusion channel (6) in such a manner that the annular material flow flows through the tubular surface of the filtering unit (7), characterized in that the filtering unit (7) has at least three layers (7 a, 7 b, 7 c), and the outer layer (7 a) and the inner layer (7 c) of the filtering unit are substantially stationary and have between them a filtering layer (7 b) that can be moved, and the outer layer (7 a) and the inner layer (7 c) have openings (10, 11) and the material is arranged to flow sideways in the filtering layer (7 b), whereby it is filtered and the filtered material remains in the filtering layer (7 b) on the side of the openings (10) of the layer (7 a) first in the flow direction of the material, and to clean the filtering unit (7), the filtering layer (7 b) can be moved in such a manner that after the move the filtered material settles at the openings (11) of the layer (7 c) last in the flow direction of the material and due to the material flow, the filtered material particles flow out of the extruder.
 5. An extruder as claimed in claim 4, characterized in that the tubular filtering unit (7) is cylindrical.
 6. An extruder as claimed in claim 4, characterized in that the tubular filtering unit (7) is conical.
 7. An extruder as claimed in claim 6, characterized in that the tubular filtering unit (7) is in the shape of an expanding cone.
 8. An extruder as claimed in any of the claims 4 to 7, characterized in that the openings (10) of the first layer (7 a) in the flow direction of the material are convergent.
 9. An extruder as claimed in any of the claims 4 to 8, characterized in that the openings (11) of the last layer (7 c) in the flow direction of the material are expanding.
 10. An extruder as claimed in any one of claims 4 to 9, characterized in that the openings (10, 11) are elongated.
 11. An extruder as claimed in claim 10, characterized in that the elongated openings are arranged in an oblique position with respect to the axial direction of the filtering unit (7).
 12. An extruder as claimed in any one of claims 4 to 11, characterized in that the filtering layer (7 b) is made of filter material.
 13. An extruder as claimed in claim 12, characterized in that the filter material is sintered metal.
 14. An extruder as claimed in any one of claims 4 to 11, characterized in that the filtering layer (7 b) is made of a solid material, and on at least one side of the filtering layer (7 b), there is a clearance (16) through which the filtered material can flow, but which clearance (16) is so small that the particles to be filtered cannot flow through it.
 15. An extruder as claimed in any one of claims 4 to 14, characterized in that the means for extruding material comprise at least one conical stator (1, 2, 8) and at least one conical rotor ((3, 3 a, 3 b).
 16. An extruder as claimed in claim 15, characterized in that the extruder has at least two stators (1, 2, 8), and there is a conical feed gap (4, 4 a, 5, 5 a) on both sides of the rotor (3, 3 a, 3 b), and at least one return channel (19) is arranged to the end part of the rotor (3, 3 a, 3 b), through which part of the material fed through the feed gaps (4, 4 a, 5, 5 a) can flow backwards.
 17. An extruder as claimed in claim 16, characterized in that a ring channel (18) is arranged to the end part of the rotor (3, 3 a, 3 b), through which material can flow to the return channel (19).
 18. An extruder as claimed in claim 4 to 17, characterized in that the filtering unit (7) is made to be replaced in its entirety as one piece. 